1. Field of the Invention
The present invention relates to a dynamic microphone having a structure and a circuit for reducing vibration noise, the circuit including a piezoelectric element having a certain capacitance as a vibration sensor for cancelling undesired vibration.
2. Related Background Art
A dynamic microphone includes a diaphragm and a voice coil fixed to the diaphragm, where the voice coil is disposed in a magnetic gap in a magnetic circuit including a magnet, a yoke, and a pole piece. The diaphragm vibrates upon receiving sound waves, and the voice coil also vibrates to traverse a magnetic flux in the magnetic gap. As a result, sound signals corresponding to the sound waves are output from the voice coil.
A hand-held dynamic microphone tends to generate unpleasant noise due to vibration caused by rubbing a microphone case with a hand, gripping or placing the microphone, or any other handling. The vibration noise is resulted from relative movement of a movable portion including the diaphragm and the voice coil to a fixed portion including the microphone case and components of the magnetic circuit, the movement being induced by vibration caused by factors other than the sound waves. Nondirectional dynamic microphones have been widely used for hand-held dynamic microphones because of its relatively low susceptibility to such vibration noise.
In the meantime, unidirectional dynamic microphones have requests about their use as hand-held dynamic microphones. Directional analysis of a unidirectional dynamic microphone indicates that its directional characteristics include a nondirectional component and a bidirectional component. The bidirectional component of the unidirectional dynamic microphone is to be mass-controlled, and thus the resonant frequency of the diaphragm is set to 100 to 200 Hz, close to the lower limit of a sound pickup range. FIG. 10 shows a vibration equivalent circuit of a typical unidirectional dynamic microphone. In FIG. 10, symbol m0 denotes mass of a diaphragm, symbol s0 denotes stiffness of the diaphragm, symbol r1 denotes acoustic resistance for obtaining the nondirectional component, and symbol m1 denotes acoustic mass for obtaining the bidirectional component. These circuit constants are set to appropriate values to fix the resonant frequency to a target value.
In the unidirectional dynamic microphone, the resonant frequency of the diaphragm is set to a value close to the lower limit of the sound pickup range as described above, and thus handling of the microphone can readily generate vibration noise called handling noise. A light material such as a copper clad aluminum wire (CCAW) is used for the voice coil to reduce mass of the movable portion, leading to a reduction in vibration noise. This, however, raises the resonant frequency of the diaphragm; hence, the stiffness of the diaphragm must be decreased (compliance must be increased) to lower the low frequency limit to a desired value. While some microphones have shock mounts to support a microphone unit therethrough as a measure against the vibration noise, the low-frequency vibration noise cannot be readily reduced only by the shock mounts.
In this way, the vibration noise cannot be readily reduced by material design or by the shock mounts within a basic frame of the microphone, and thus an additional structure or mechanism is necessary to actively reduce or cancel the vibration noise. Conventional microphones having such a structure or mechanism are described below.
Japanese Unexamined Patent Application Publication No. 11-331987 (JP-A-11-331987) discloses a microphone including a microphone unit, a microphone case that supports the microphone unit therein through shock mounts, a shock sensor that detects vibration applied to the microphone case, and a signal processor that attenuates signals generated through electroacoustic conversion in the microphone unit and outputs the attenuated signals.
Japanese Unexamined Patent Application Publication No. 10-145882 (JP-A-10-145882) discloses a microphone including two vibration transmission paths, that is, a diaphragm-side path and a magnetic-circuit-side path, where a difference in relative velocity between a diaphragm and a magnetic circuit is reduced to cancel the vibration noise. Specifically, first and second elastic members are disposed on the top and the bottom of the periphery of the diaphragm 13, respectively, and the diaphragm 13 is attached to a unit case through the first elastic member at the top of the periphery, and a magnetic circuit section is accommodated in a unit case while one end of the circuit section is in contact with the second elastic member at the bottom of the periphery. Vibrations applied to the unit case are primarily transmitted to the second elastic body through the first elastic member and then to the magnetic circuit section. Such a limitation of the solid propagation path for the vibration waves can reduce the vibration noise over a wide range caused by rubbing of the unit case.
Japanese Unexamined Patent Application Publication No. 11-196489 (JP-A-11-196489) discloses a dynamic microphone including a dynamic vibration pickup for detecting vibration noise, where vibration noise generated in a microphone unit is cancelled using detected signals. Specifically, a vibration detection unit being an acoustic element is disposed in an air chamber communicating with the microphone unit, and variation of internal pressure in the air chamber, which is caused by vibration of a diaphragm of the vibration detection unit, propagates to the back of another diaphragm of the microphone unit so as to prevent the second diaphragm from being displaced by external vibration, leading to active cancellation of the vibration noise.
In the microphone disclosed in JP-A-11-331987, the output signal level of the microphone is attenuated in response to external vibration so as to attenuate the output noise level. A disadvantage of the microphone is a reduction in output signal level in response to the external vibration.
In the microphone disclosed in JP-A-10-145882, the diaphragm and the magnetic circuit move parallel in response to external vibration to reduce a difference in relative speed therebetween. Thus, in order to reduce the vibration noise on target, mechanical impedances, including mass of movable portions such as the diaphragm, mass of the magnetic circuit, and an elastic coefficient of each elastic member, must be designed and adjusted to appropriate values. Such design and adjustment requires much time and effort. In addition, these coefficients or values may vary due to variations in environmental conditions such as temperature, precluding a reduction in the vibration noise on target.
In the dynamic microphone disclosed in JP-A-11-196489, the dynamic vibration pickup must be designed and manufactured such that the vibration pickup produces output in accordance with the vibration noise in the microphone unit, which requires much time and effort for design and adjustment. Moreover, mechanical impedances must be designed and adjusted to appropriate values as in JP-A-10-145882, which also requires much time and effort for design and adjustment. Furthermore, the mechanical impedances may vary due to variation in environmental conditions such as temperature, making it difficult to reduce the vibration noise on target.